Precipitated silicas find use in a broad range of manufactured products ranging from cosmetic and food products to industrial coatings and elastomeric materials, such as tires. Silicas are particularly useful in dentifrice products (such as toothpastes) where they function as fillers, abrasives, and thickeners. Because of this functional versatility, and also because silicas have a relatively high compatibility with active ingredients like fluoride compared to other dentifrice abrasive (notably alumina and calcium carbonate), there is a strong desire among toothpaste and dentifrice formulators to include them in their products.
However, it can be difficult to incorporate abrasive silicas into transparent dentifrice products, which have become increasingly popular in recent years because of their greater appeal to some consumers and because they allow manufacturers to impart increased distinctiveness to their product. This formulation difficulty arises because in order to produce a silica-containing transparent toothpaste, it is necessary to ensure that the refractive index of the silica closely matches the refractive index of the toothpaste matrix, that the silica has a high degree of light transmittance, and that the silica has sufficient abrasivity to provide cleaning of the tooth surfaces, when incorporated into a dentifrice.
The requirement that the refractive index of the silica match the refractive index of the toothpaste generally means that the concentration of water in the toothpaste must be maintained at relatively low levels. Water generally has a far lower refractive index than silica (commercially available silicas have a refractive index of 1.438 to 1.451, while water has a refractive index of 1.332), and so as the toothpaste's water concentration increases, the refractive index of the toothpaste decreases. Accordingly, in order for the refractive index of the silica to match the refractive index of the toothpaste, the water concentration in the toothpaste must be minimized. This is undesirable because water is generally the least expensive component of a toothpaste, and decreases in water concentration are normally offset by increases in humetctant concentration (which is quite expensive). Thus, decreasing water concentration will cause a corresponding increasing in the toothpaste unit cost.
Thus, a toothpaste formulator striving to produce a transparent gel toothpaste must balance several factors. Silica is an indispensable ingredient to produce a toothpaste that is effective at cleaning teeth, but adding silica can reduce the transparency of the overall toothpaste product because of its low degree of transmittance and high refractive index. So while adding silica provides cleaning benefits, the silica's high refractive index requires a decrease in the water concentration and a concomitant increase in the humectant concentration, resulting in a significant increase in product cost.
Given the foregoing, there is a continuing need for a silica composition that not only provides excellent abrasive performance, but also has a relatively high degree of transmittance, and an index of refraction that is sufficiently low, such that the silica can be included in a transparent toothpaste composition having a relatively high concentration of water.